A paleoenvironmental time-series spanning the Holocene was constructed using 29 radiocarbon ages and 149 standardized δ13Csom values from alluvial terrace profiles along the middle Delaware River valley. There is good agreement between increasing δ13Csom and Panicoideae phytolith concentrations, suggesting that variations in C4 biomass are a major contributor to changes in the soil δ13C. A measurement error deconvolution curve over time reveals two isotope stages (II–I), with nine sub-stages exhibiting variations in average δ13Csom (average %C4). Stage II, ~ 10.7–4.3 ka, shows above-average δ13Csom (increase %C4) values with evidence of an early Holocene warming and dry interval (sub-stage IIb, 9.8–8.3 ka) that coincides with rapid warming and cool-dry abrupt climate-change events. Sub-stage IId, 7.0–4.3 ka, is an above average δ13Csom (increase %C4) interval associated with the mid-Holocene warm-dry hypsithermal. The Stage II–I shift at 4.3 ka documents a transition toward below average δ13Csom (decrease %C4) values and coincides with decreasing insolation and hydroclimatic change. Sub-stages Ib and Id (above average %C4) coincide with the first documented occurrence of maize in the northeastern USA and a substantial increase in human population during the Late Woodland. These associations suggest that people influenced δ13Csom during the late Holocene.

Holocene and modern travertine formed in spring-fed Havasu Creek of the Grand Canyon, Arizona, was studied to determine the factors governing its oxygen-isotope composition. Analysis of substrate-grown travertine indicates that calculated calcite-formation temperatures compare favorably with measured water temperatures, and include silt-rich laminae deposited by monsoon-driven floods. Ancient spring-pool travertine is dated by U-series at 7380 ± 110 yr and consists of 14 travertine-silt couplets of probable annual deposition. One hundred eighty high-resolution δ18O analyses of this mid-Holocene sample average −11.0‰ PDB. The average value for modern travertine is ∼0.5‰ lower, perhaps because mid-Holocene temperature was higher or there was proportionally greater summer recharge. δ18O cyclicity in the mid-Holocene travertine has average amplitude of 1.9 ± 0.5‰ PDB, slightly less than the inferred modern-day annual temperature range of Havasu Creek. The annual temperature range might have been reduced during the 14-yr interval compared to present, although other non-temperature factors could account for the muted annual variation. Silt-rich laminae within isotopically lower calcite in the modern and mid-Holocene travertine verifies the seasonal resolution of both samples, and suggests that similar temperature-precipitation conditions, as well as monsoon-generated summer floods, prevailed in the mid-Holocene as they do throughout the Grand Canyon region today.